This invention relates to a process for converting a metal carbide to carbon on a surface of a monolithic part that is predominantly a metal carbide or has predominantly a metal carbide surface, e.g., coating, by etching in a halogen-containing gas and more particularly, the uses of such materials having a converted carbon surface layer in environments that require long wear and low friction.
The extraction of silicon from silicon carbide powders and fibers by halogens (F2, Cl2, Br2, I2 or mixtures) or compounds containing one or more halogens (e.g., HF, CCl4, and the like) can lead to the formation of free carbonxe2x80x94see Gogotsi, et al. xe2x80x9cCarbon coatings on silicon carbide by reaction with chlorine-containing gasesxe2x80x9d, J. Mater. Chem., pp. 1841-1848 (1997); and Gogotsi, et al. xe2x80x9cFormation Of Carbon Coatings On SIC Fibers By Selective Etching In Halogens And Supercritical Waterxe2x80x9d, 22nd Annual Conference on Composites, Advanced Ceramics, Materials, and Structures: A, Ceramic Engineering and Science Proceedings, Vol. 19, December 1998. This method can be used to obtain carbon from SiC, TiC, WC and other carbides to form volatile halides (SiCl4 and TiCl4 are typical examples), but do not form soluble oxides. This process can be considered as a reaction of the following type:
MC(s)+D(g)=C(s)+MyDx(g)+Dz(g),
where MC is a metal carbide, D is a gaseous halogen or halogen-containing etchant (e.g., Cl2, HCl, and the like), MyDx is a gaseous fragment reaction product species, and Dz is another possible fragment reaction product species. This type of reaction can lead to the formation of free carbon, that may maintain the sp3 structure which it has in carbide and form diamond. Alternatively, it can be transformed to graphite, or form various amorphous or disordered carbon structures intermediate to diamond and graphite.
Specifically, for the preferred metal carbide, silicon carbide, the chlorination reactions:
SiC(s)+2Cl2(g)=SiCl4(g)+C(s) and
SiC(s)+4HCl(g)=SiCl4(g)+C(s)+2H2(g)
lead to the formation of a carbon surface, or a carbon coating, on the surface of metal carbides or to the complete transformation of carbide particles into carbon. This is due to the fact that a metal-halogen, e.g., SiCl4, is much more thermodynamically stable than a carbon-halogen, e.g., CCl4, at elevated temperatures, so that chlorine reacts selectively with the Si at SiC surfaces leaving carbon behind. The structure of the carbon layer is affected by temperature and by the composition of the chlorinating gas mixture. In accordance with the present invention carbon films or surface layers were formed as an integral part of monolithic metal carbide parts on the surface of commercially available monolithic xcex1-SiC and xcex2-SiC specimens by high temperature (100xc2x0 C. or greater) chlorination at atmospheric pressure in H2xe2x80x94Cl2 and Cl2 gas mixtures, using an inert gas, such as Argon, to dilute the halogen gas content to a desired concentration.
Commercial methods of synthesis of carbon coatings have serious limitations. The CVD method does not allow the synthesis of coatings on powders and other particulate materials. Heteroepitaxial growth of diamond by CVD still has its problems. Generally CVD and Physical Vapor Deposition (PVD) processes exhibit low rates of deposition and require a nucleation pre-treatment for diamond synthesisxe2x80x94see Yang, U.S. Pat. No. 5,298,286xe2x80x94March 1994. Plasma-assisted CVD is especially slow and energy-consuming techniquexe2x80x94see Kieser, U.S. Pat. No. 4,661,409xe2x80x94April 1987, and U.S. Pat. No. 4,569,738xe2x80x94February 1986. Moreover, carbon films deposited with the CVD and PVD methods do not generally adhere to the substrate, often peeling off during loading which can take place in tribological applications of carbon coatings, such as in cutting tools, bearings, seals, and the like. Special techniques used to improve adhesion between diamond films and other ceramic substrates have not been completely successful, particularly on WC tools because of the large differences in physical properties between diamond and WC. Other methods that have been used to produce carbon films with special properties include laser vaporizationxe2x80x94see Mistry, U.S. Pat. No. 5,731,046xe2x80x94March 1998; high temperature synthesis, sputteringxe2x80x94Pulker, U.S. Pat. No. 4,173,522xe2x80x94November 1979; pyrolisisxe2x80x94Beatty, U.S. Pat. No. 4,016,304xe2x80x94April 1977, Bokros, U.S. Pat. No. 3,977,896xe2x80x94August 1976, Araki, U.S. Pat. No. 3,949,106xe2x80x94April 1976; decomposition of organic materials and ion beam depositionxe2x80x94Dearnaley, U.S. Pat. No. 5,731,045; 5,725,573xe2x80x94March 1998, U.S. Pat. No. 5,512,330xe2x80x94April 1996, U.S. Pat. No. 5,393,572xe2x80x94February 1995, U.S. Pat. No. 5,391,407xe2x80x94February 1995, Cooper U.S. Pat. No. 5,482,602xe2x80x94January 1996, Bailey, U.S. Pat. No. 5,470,661xe2x80x94November 1995, and Malaczynski, U.S. Pat. No. 5,458,927xe2x80x94October 1995. Most of these processes are expensive and energy-intensive. Additionally, there is no single versatile method that could provide all types of carbon coatings.
Another process which could be used to synthesize carbon coatings on the surface of carbides is by hydrothermal leachingxe2x80x94see Gogotsi, Ukrainian Patent 10393A; and Yoshimura, Japanese Patent 07,232,978. Both graphitic carbon and diamond can be obtained by interaction of SIC with water. However, the hydrothermal method can only be applied to carbides that form soluble or volatile hydroxides, such as Si(OH)4. Additionally, the use of high-pressure autoclaves in hydrothermal synthesis can make scaling up of hydrothermal reactors difficult and expensive.
In brief by treating a metal carbide, preferably a monolithic part that is predominantly a metal carbide or has predominantly a metal carbide surface, e.g., coating, with a gaseous halogen, in accordance with the principles of the present invention, carbon coatings or surface layers are obtained that have an improved interfacial strength compared to prior art methods because the carbon is not deposited from the environment, rather the carbide surface is converted into carbon. This leads to an improved adhesion and decreased (or elimination of) delamination of the carbon coating.
Low cost: virtually any possible carbon structure (amorphous carbon, graphite, diamond, diamond like carbon, and the like) can be obtained on the surface of commercially available metal carbides by simply tuning the gas composition and/or temperature. This allows the use of the same reactor vessel, of almost any volume, for all types of carbon coatings.
The process of the present invention can be carried out at atmospheric pressure and does not require plasma or other high-energy sources.
Impure raw SiC can be used in accordance with the present invention containing predominantly (more than 50% by weight) SiC, preferably at least 80% SiC, because halogenation will remove most metallic impurities from the predominantly SiC material, thus improving the purity of SiC.
Unlike CVD, not only carbide components or fibers can be coated, but also powders, whiskers and platelets.
Monolithic parts with complex shapes and surface morphologies can be xe2x80x9ccoatedxe2x80x9d in accordance with the present invention. It is the reaction gas at atmospheric pressure in contact with the carbide surface that transforms the material. This is important for two reasons. First, the reaction can proceed anywhere the gas can reach, e.g., crevices, channels, intricate layered surfaces, holes, etc. This would be impossible with most of the other current available processes. Second, it allows control of growth on the atomic level.
Templating of the surface structure/coating will allow various ore sizes (angstroms to nanometers).
The process of the present invention is environmentally friendly technology since it can be operational as a closed loop process with the recovery of Si by decomposition of the metal halide, e.g., SiCl4, and returning the halogen gas to the manufacturing process.
One object of the present invention is to provide a new method of producing carbon layers at low cost on metal carbides with virtually any possible carbon structure (amorphous carbon, graphite, diamond, diamond like carbon, etc.) can be obtained on the surface of commercially available SiC (and other metal carbides).
Another object of the invention is to provide improved interfacial strength between the carbon layer and the substrate metal carbide compared to other methods.
A further object of the invention is to provide a new method of carbon layer formation (coating) that allows the use of the same reactor vessel, of almost any volume, for all types of coatings desired.
Yet another object of the invention is to provide a new method of carbon coating on metal carbides at atmospheric pressure vs. higher pressures or lower pressures (vacuum) (which are most dangerous and expensive, and slow).
Still yet another object of the invention is to provide a new method of carbon coating on metal carbides that does not require plasma or other expensive high-energy sources.
Another object of the invention is to allow the use of impure, raw SiC (or other metal carbide) to be used (hence save money) in the production of coated metal carbides.
Another object of the invention is to allow not only carbide components or fibers to be coated, but also powders, whiskers and platelets (not possible with current processes, CVD, PVD, and the like).
A further object of the invention is to allow parts with complex shapes and surface morphologies to be coated, e.g., crevices, channels, intricate layered surfaces, holes, etc.
Yet another object of the invention is to allow control of carbon coating growth on the atomic level.
Still yet another object of the invention is to allow variation in pore sizes (angstroms to nanometers) of carbon coatings to tailor surface properties.
Another object of the invention is to provide a new environmentally friendly technology for coating of metal carbides.